Directional drilling

Directional drilling, also known as slant drilling, involves creating boreholes that are not vertical. The practice is divided into four main categories: oilfield directional drilling, utility installation directional drilling, directional boring (also called horizontal directional drilling or HDD), and surface in seam (SIS) drilling, which horizontally targets a vertical bore to extract coal bed methane.

The technology took off after the late 1920s, when lawsuits over wells allegedly crossing property lines forced the oil industry to accept that wells didn't have to be vertical. Early court cases relied on indirect evidence like production changes in nearby wells, but these disputes spurred the creation of small tools to survey wells during drilling. Today, horizontal directional drill rigs are evolving toward larger scale, micro-miniaturization, mechanical automation, the ability to work in hard strata, and record-setting length and depth with monitored drilling.

Measuring a wellbore's inclination—how far it tilts from vertical—is straightforward with a pendulum. Finding its azimuth, or direction relative to the geographic grid, proved trickier. Magnetic fields could work but were distorted by metal in the wellbore and drilling equipment. The breakthrough came when Sperry Corporation, under contract to Sun Oil (involved in one of those lawsuits), adapted its small gyroscopic compasses—originally made for aircraft navigation—for wellbore use. This led to the spin-off Sperry Sun, a brand now owned by Halliburton. To determine a wellbore's position at any point, three measurements are taken: the measured depth along the borehole, the inclination, and the magnetic azimuth. Together, these form a "survey," and a series of surveys tracks the well's progress and location.

Earlier rotary drilling had shown how certain bottom hole assembly (BHA) configurations tended to "drill crooked hole," making accidental deviations worse. Conversely, early directional drillers (DDs) learned BHA designs and practices that could steer a crooked hole back toward vertical.

In 1934, H. John Eastman and Roman W. Hines of Long Beach, California, along with George Failing of Enid, Oklahoma, became directional drilling pioneers by saving the Conroe, Texas, oil field. Failing had recently patented a portable drilling truck, starting his company in 1931 by mounting a rig on a truck with a power take-off. This allowed rapid drilling of slanted wells, and the ability to quickly drill multiple relief wells was key to releasing gas pressure and extinguishing the Conroe fire. A May 1934 *Popular Science Monthly* article noted that only a handful of people worldwide could make a drill bit "snake its way in a curve" a mile underground to hit a target. Eastman Whipstock, Inc., became the world's largest directional drilling company in 1973.

Survey tools and BHA designs made directional drilling possible, but it was seen as obscure. The next big advance came in the 1970s with downhole drilling motors (mud motors), powered by the hydraulic force of drilling mud circulating down the drill string. These let the bit keep rotating at the bottom while most of the drill pipe stayed still. A bent sub—a piece of bent pipe between the stationary pipe and the motor—allowed the wellbore's direction to change without pulling out the entire drill string to insert a whipstock. Combined with measurement-while-drilling tools (which use mud pulse telemetry, wired pipe, or electromagnetic telemetry to send directional data to the surface without stopping drilling), directional drilling became much easier.

Some wellbore profiles are hard to drill while the pipe rotates. Using a downhole motor for directional drilling means occasionally stopping rotation and "sliding" the pipe through the channel as the motor cuts a curve. Sliding can be tough in certain formations and is almost always slower—and thus more expensive—than rotating the pipe. So, the ability to steer the bit while the pipe rotates is highly desirable. Several companies have developed rotary steerable systems (RSS) for this purpose, allowing access and directional control in formations that were previously unreachable or uncontrollable.

Directional drilling serves several purposes: it increases the exposed section of the reservoir by drilling through it at an angle; it reaches reservoirs where vertical access is difficult or impossible, such as under a town, a lake, or a hard-to-drill formation; and it allows multiple wellheads to be grouped together on a single surface location.